In the 1960s, Nowotny and his co-authors have prepared many ternary carbides or nitrides and studied their structural characteristics (H. Nowotny, Prog. Solid State Chem., 2, 27 (1970)). These ternary carbides and nitrides are expressed by a general formula of Mn+1AXn, where M is a transition metal, A is a main group (mostly IIIA and IVA) element, X is either C or N element, and n is 1, 2, or 3. The ternary layered compounds, Mn+1AXn, are abbreviated to MAX phases (M. W. Barsoum, Prog. Solid State Chem., 28, 201 (2000)). The ternary layered compounds all have hexagonal structures which can be described as octahedron being interleaved with atoms layers, and their physical and mechanical properties are closely related to the corresponding carbides and nitrides.
The common characteristic of MAX phases is that they are comprised of covalent bonding, metallic bonding and ionic bonding. So, MAX phases possess combination properties of both ceramics and metals, such as good electrical and thermal conductivity, machinability, excellent thermal shock resistance, high modulus, and high specific strength and so on. In MAX phases, TiAX phases, including Ti3SiC2, Ti2AlC, Ti2AlN, and Ti4AlN3, have been extensively investigated.
In the early 1960s, Jeitschko et al. (W. Jeitschko, H. Nowotny, F. Benesovsky, Monatsh. Chem., 95, 1198(1963)) discovered and deciphered the cell structure of Ti2AlN. In the mid 1970s, Ivchenko et al. (V. I. Ivchenko, M. I. Lesnaya, V. F. Nemchenko, T. Y. Kosolapova, Porosh. Metall., 160, 60 (1976); V. I. Ivchenko, T. Y. Kosolapova, Porosh. Metall., 150, 1 (1975); V. I. Ivchenko, M. I. Lesnaya, V. F. Nemchenko, T. Y. Kosolapova, Porosh. Metall., 161, 45 (1976); V. I. Ivchenko, T. Y. Kosolapova, Porosh. Metall., 164, 56 (1976)) fabricated Ti2AlN bulk sample with a density of 90% to 92% and measured some of their properties. Ivchenko et al. reported that microhardness values were higher than 20 GPa, comparable to the hardness of binary nitride. This is much higher than the hardness (4 GPa) of Ti2AlN subsequently reported by Barsoum et al. (M. W. Barsoum, M. Ali, T. El-Raghy, Met. Mater. Trans., 31A, 1857 (2000)). With reference to the other properties of TiAX phases, it can be deduced that the properties of Ti2AlN reported by Barsoum et al. are very close to the truth. Although Ti2AlN is the first ternary nitride discovered and extensively researched, it is relatively difficult to produce pure Ti2AlN. The Ti2AlN sample made by Barsoum et al. also includes three other phases (11% to 20% by volume), in addition to the Ti2AlN matrix, which are Al2O3, Ti3P, and a phase with a Ti:Al:N ratio of 4:1:3. Despite their best efforts, the sample of this “413” phase cannot be eliminated. The best samples contained 10% to 15% “413” phase. This “413” phase is not Ti4AlN3 (A. T. Procopio, T. El-Raghy, M. W. Barsoum, Met. Mater. Trans., 31A, 373 (2000)) that was reported as Ti3Al2N2 phase in 1984 by Schuster and Bauer (J. C. Schuster, J. Bauer, J. Solid State Chem., 53, 260 (1984)).
Ti2AlN is a ternary nitride ceramic of hexagonal-close-packed structure, with Ti at 4f, Al at 2c positions, and N filling the interspaces of Ti octahedron. The methods of synthesizing Ti2AlN reported so far include solid reaction method (M. W. Barsoum, M. Ali, T. El-Raghy, Metall. Mater. Trans., 31A, 1857 (2000)) and magnetron sputtering method (T. Joelsson, A. Horling, J. Birch, L. Hultman, App. Phy. Lett. 86, 111913 (2005)). Ti2AlN bulk sample with 80% (by volume) Ti2AlN phase was obtained after reactive hot-isostatic pressing (HIP) at 1400° C. under a pressure of 40 MPa for 48 hours, with micron-scale pure Ti powder and AlN powder, in Barsoum's method. But Barsoum's preparation method had some shortcomings such as high reactive temperature, long time, large grain size of the bulk and the like. For the magnetron sputtering method, relatively pure Ti2AlN thin film can be obtained, but it is difficult to produce bulk materials and the yield is relatively low. Ti2AlN is a potential candidate of high-temperature structure materials for use in the temperature range of 1000° C. to 1300° C. However, the oxidation behavior follows the parabolic law in the range of 1000° C. to 1100° C. for time shorter than 20 h; for time longer than 20 h, the oxidation behavior deviates from the parabolic law and becomes linear law. Thus, the high-temperature oxidation properties are unsuitable for the use at a temperature range of 1000° C. to 1300° C. for a long time. So it is necessary to improve the high-temperature oxidation properties of Ti2AlN.
Al2O3 is an ionic oxide with slight distortion hexagonal-close-packed structure, with O2− at hexagonal-close-packed lattice position, and Al3+ filling the interspaces of O2− octahedron. This structure remains relatively stable up to the melting point, so the working temperature of Al2O3 can approach to 1800° C. There is almost no loss or gain in weight when Al2O3 is used in an air atmosphere. Moreover, Al2O3 and Ti2AlN are very close in density and thermal expansion coefficient, are compensated each other in hardness and compressive strength. Al2O3 is chosen to strengthen Ti2AlN matrix, which can improve the high-temperature oxidation properties at the same time. Table 1 shows the main properties of Al2O3 and Ti2AlN.
TABLE 1Physical and mechanical properties of Ti2AlN and Al2O3PropertiesTi2AlNAl2O3Density (g/cm3)4.31   3.9Vickers Hardness (GPa)4 26Compressive Strength (MPa)4502945 Electrical Resistivity (μΩ · m)0.25, 0.312   >1018Thermal Expansion Coefficient (K−1)8.2 × 10−68.3 × 10−6Young's Modulus (GPa)—380386Shear Modulus (GPa)—175Melting Point (° C.)—2054 
Generally, Al2O3 dispersion-strengthened Ti2AlN composites are compacted by hot pressing or hot isostatic pressing via powder metallurgy method. There are the following several methods to proportion powders:
1. The use of Al2O3 powders and Ti2AlN powders, which belongs to the type of no in situ reaction;
2. The use of Al2O3 powders and raw powders to form Ti2AlN, which belongs to the type of in situ reaction to form Ti2AlN;
3. The use of raw powders, which belongs to the type of in situ reactions to form Al2O3 and Ti2AlN.
For the first and second method, there are problems such as non-homogenous distribution, easy agglomeration and significant particle growth of Al2O3. These problems become more severe with increasing volume fraction of Al2O3. For the third method, the in situ formed Al2O3 particles are fine and uniformly dispersed and the volume fraction of Al2O3 can be controlled up to 50%.
In order to obtain in situ formed Al2O3 particles dispersion-distributed in the Ti2AlN grains formed by an in situ reaction, powders are produced by a hydrogen plasma-metal reaction method (HPMR). HPMR is suitable for industrial production for use of the preparation of nanoparticles of various metals and alloys. The fundamental principle of HPMR is a process in which atoms are changed from a liquid state to a gas state by using a plasma heat source In the gas state, atom clusters collide with inert gas to transfer energy and make gas clusters cool quickly, producing nanoparticles. During the development of HPMR, Wada (N. Wada, Jpn. J. Appl. Phys., 6, 553 (1967); N. Wada, Jpn. J. AppI. Phys., 7, 1287 (1968); N. Wada, Jpn. J. Appl. Phys., 8, 551 (1969)) first investigated the effects of pressure and kinds of gases on the particle size of as-prepared powder and found that the addition of hydrogen to atmosphere can accelerate the evaporation of metals. Uda et al. (M. Uda, Bull. Japan Inst. Metals., 22, 412 (1983); S. Ohno, M. Uda, J. Japan Inst. Metals., 48, 640 (1984); S. Ohno, M. Uda, J. Japan Inst. Metals., 53, 946 (1989)) extended Wada's method by using a mixture of hydrogen and argon gas with a total pressure of 1 atm, and by substituting the plasma jet gun with an general tungsten electrode. These measures improved the yield of nanopowders from laboratory scale to industrial production. Uda et al. (S. Ohno, M. Uda, J. Japan Inst. Metals., 48, 640 (1984)) found in their study that the driving force for generation of nanoparticles from metals is closely related to these factors such as the heat of evaporation of metals, the heat of formation for recombination from atoms (dissolved in metal) to molecules (evolved into gas phase) of hydrogen and melting points of metals. Ohno and Uda (S. Ohno, M. Uda, J. Japan Inst. Metals., 53, 946 (1989)) studied the evaporation rules of a series of pure metals, and investigated the preparation and characteristics of nanoparticles of Fe—Ni, Fe—Cu and Fe—Si binary alloys. The Chinese Patent Application No. 03133805.4 investigated the synthesis of nanoparticles of Ti-Al binary system. The research disclosed that the concentration of Al of the nanoparticles deviate positively from that of the master alloys. Components of nanoparticles to be used for the preparation of the composites have been designed according to research results of the patent.
From the above, we know that it is relatively difficult to synthesize Ti2AlN. The available technologies are mainly solid reaction methods, and the recently reported magnetron sputtering method was used to synthesize Ti2AlN thin film but not bulk material. Little research has been performed on Ti2AlN due to the difficulty in synthesis. Up to now, there has been no report on the Ti2AlN composites strengthened by Al2O3.